Proposals have already been made for using microwave energy to reticulate polymers that contain polar constituents.
Such microwave heat treatment methods have the particular advantages of requiring a relatively short treatment time (generally a few minutes), of high energy efficiency, and of an average forming temperature which is lower than that obtained using conventional heat treatment methods.
In particular, French published patent specification number 2 458 323 describes a method and an apparatus for using microwaves to treat coatings on substrates.
Further, it is known that material is reticulated particularly efficiently by microwaves when the material is in thin layer form, thereby obtaining a "filmogen" type binder.
However, it turns out in practice that the methods of applying microwave energy proposed up to now are very poorly adapted to processing certain materials.
This is particularly true of coatings having a high concentration of electrically conductive material therein.
It is recalled that if microwaves are applied to thick material of high conductivity, the material is generally not heated since these very properties of the material impose an electric field which is either zero or else orthogonal to the surface of the material.
In contrast, when the thickness of the conductive material is of the same order as the thickness of the skin effect (generally denoted as the "penetration depth .delta."), and if the charge density is high, the material heats very fast and may even burn in a few seconds at a critical very high density.
In particular, iridescence may appear on the conductive material if the heat generated cannot be removed quickly enough by exchange with the support or with air.
It is recalled that the tangential component of the electric field falls off exponentially in a conductor in the vicinity of its surface, and that the term "penetration depth .delta." or "skin effect thickness" are used to designate the depth at which the electric field is reduced to 1/2.718 (ie. 1/e) of the initial value of the field at the surface of the conductor.
In parcticular, tests on an ink comprising a polymerizable binder such as alkyd melamine resin with conductive particles of carbon or of a metal such as copper or aluminum embedded therein, have shown that when a film of the ink is deposited on a dielectric support which is placed parallel to the electric field, the film may be destroyed before the binder has had time to polymerize.
This is due to the fact that in conductive materials which are of the same order of thickness as the skin effect depth, the average of the tangential component of the electric field is no longer negligible, thus greatly increasing the heating effect which is proportional to the square of the electric field as well as to the conductivity of the material.
Further, reducing the heating of the material to be treated merely by reducing the intensity of the electric field applied to the dielectric support on which the material is coated, or by reducing the power of the propagated wave, is not acceptable either. This tactic has the accompanying drawback of also reducing the heating of dielectric support, which is generally to be avoided since that may lead to poor adherence of the material on the support due to the support remaining cold during the heat treatment.
It is for these reasons that conventional techniques still have to be used for the heat treatment of various materials, and in particular coatings of high conductivity, in spite of the numerous advantages which can otherwise be obtained by using microwaves for the heat treatment of materials, and in particular surface coatings.
Preferred implementations of the present invention provide a new method of applying microwave energy to a coating deposited on a dielectric support, which method enables a material such as an electrically conductive ink to adhere satisfactorily to the support without deteriorating the material itself.